The present invention relates to a new and improved method of manufacturing an optically structured filter and to a new and improved optically structured filter.
In its more particular aspects, the present invention relates specifically to a new and improved method of manufacturing an optically structured filter with essentially smooth surface, in particular to a photolithographic mask for production of microelectronic devices, and which filter comprises at least one pattern with differing transmission for the electromagnetic radiation to be filtered.
The known methods of manufacturing optically structured filters for electromagnetic radiation, in particular such filters or photolithographic masks for the production of microelectronic devices, all require a number of method steps which include wet chemical developing procedures as well as etching steps and/or reactive ion etching or "ion milling".
In one known method, a metal or metal oxide layer with a thickness of 80-140 nm and, opaque to the required wavelength--range of radiation is provided on a supporting material, in particular on synthetic quartz glass. On top of this layer a photoresist layer with a thickness of at least 300 nm is deposited. The photoresist layer is exposed to electromagnetic radiation (UV- or X-rays) or to a particle beam (electrons, ions), according to a predetermined pattern. This exposure to radiation induces local changes in (wet chemical) etching rate; in the case of a photoresist type known as "positive" photoresist, the previously exposed areas are preferentially removed by the etching process and the underlying optically opaque layer is accessible to a further etching step. During this second etching step the optically opaque layer is locally removed so that in a final, third etching step (reactive plasma etching or wet chemical etching of the photoresist) in which the remaining not exposed sections of the photoresist layer are removed, a working mask is finally formed and consists of a pattern of optically opaque material on an optically transparent supporting layer.
According to a relatively new process (J. Electrochem. Soc. 129, p. 827-830, 1982; B. A. MacIver), a transparent layer of a positive photoresist is deposited on a transparent supporting quartz layer. The photoresist layer is exposed to UV or electron irradiation according to a predetermined pattern. In a wet chemical developing step the exposed areas of the photoresist are etched away throughout up to the transparent supporting material. In a further step the remaining, previously not exposed, photoresist areas are transformed into a highly opaque state particularly for UV--radiation by implantation of silicon ions so that a working mask consisting of optically opaque material on top of a transparent supporting layer and to the original exposure pattern, is finally formed.
All of the manufacturing methods described so far can be classified as "lithographic processes" in which the chemical behaviour of a layer of photoresist is locally changed by different exposure processes (UV, X ray, electron, ion irradiation). These methods also require one or more wet or plasma chemical etching steps.
Apart from the number of required processing steps, these chemical etching steps are connected with a number of disadvantages: During wet chemical etching methods, "under-etching", i.e. lateral broadening of the etching action under the structured photoresist protecting from the etching fluid is relatively extensive which limits the obtainable width of structurizations to about 600-700 nm. Plasma--chemical etching methods are largely free from such under-etching, however, may cause optical diffraction effects when using the mask in photolithography due to a not completely avoidable attack on the surface of the transparent supporting layer.
A further disadvantage of the manufacturing methods described hereinabove is the fact that the finished filters or working masks do not have smooth surfaces. In fact, the surface of the optically opaque layer has recesses of a depth equal to the layer thickness in the regions corresponding to the original exposure pattern. In contrast to a filter or mask with a smooth surface, there is thus increased in a filter or mask produced according to the aforementioned hitherto known methods, the danger of contamination by small dust particles which are trapped in the recesses and which can be removed only with great difficulty. This leads to a decrease of process efficiency in the production of microelectronic circuits.